1. The Field of the Invention
The present invention relates to a portable device for detecting radiation and, more particularly, to a handheld CZT radiation detector.
2. The Relevant Technology
Radioactive materials are unstable and emit radiation in the form of alpha, beta, gamma, or X-rays. Many different types of radiation detectors have been designed and manufactured to produce data corresponding to radioactive materials.
One type of radiation detector is a pulse mode detector, in which a separate electrical pulse is generated for each individual radiation quantum (e.g., a gamma ray) that interacts with a detector. A high-purity germanium detector, which is often cooled by liquid nitrogen, is one example of a pulse-mode detector. By way of example, a gamma ray interacts with a detector surface coupled to a cathode and an anode. A portion of the energy of the gamma ray may be deposited on the detector to produce a charge. From the point of interaction, freed electrons drift towards the anode and ions (or holes) drift towards the cathode. A signal relating to the produced charge is often captured and manipulated by charge-sensitive preamplifiers and shaping amplifiers, resulting in a voltage pulse. Before entering a shaping amplifier, the pulse may have a long tail because the energy produced by the gamma-ray interaction decreases gradually. The shaping amplifier cuts off that tail, enabling detection of more pulses within a fixed period of time. The peak amplitude of such a voltage pulse is proportional to the energy deposited on a detector by a gamma ray.
Analog-to-digital converters (ADCs) are frequently employed to generate a digital number indicating the height, or the amplitude, of each voltage pulse. Such digital pulse data may be gathered and analyzed to learn more about the corresponding radioactive material. For example, the digitized pulse data may be categorized into channels, each channel indicating a specific energy level range into which the amplitude of the pulse falls. Energy levels are often measured in kiloelectron volts (KeVs). Devices that analyze multiple channels of pulse data are called multi-channel analyzers. Pulse data is often displayed on a chart showing the number of pulses (or counts) that the detector receives at a specific energy level range.
These charts frequently show a series of consecutive energy level ranges and a number of counts received in each range. This data so configured is frequently referred to as pulse height data or a pulse height distribution. By analyzing pulse height data, experts in the field may make determinations regarding the corresponding radioactive material. Such determinations may be made by automated analysis algorithms, a visual inspection, or a combination of the two.
Pulse height data is particularly useful in determining the composition of a corresponding radioactive material. Different radioisotopes emit radiation at varying energy levels. For example, plutonium-239 emits gamma radiation of approximately 203, 330, 375, 414, and 451 KeVs, among other energy levels. By examining the energy levels and intensity of such peaks within the pulse height data, the source of the radiation may be identified.
One application of this technology relates to radioisotope detection and identification to prevent illegal transportation of nuclear materials. The U.S. Customs Service, the Federal Bureau of Investigation, the U.S. Secret Service, and the International Atomic Energy Agency (IAEA.) share this common interest. The United States is particularly concerned about shipments of fissionable nuclear materials such as uranium or plutonium. Likewise, environmentalists and consumer health advocates are similarly concerned about detecting and identifying radioactive materials.
A radioisotope detector""s resolution affects its ability to accurately detect and identify radioisotopes. Ideally, pulses generated by a detector fall within discrete channels, creating tall and narrow peaks. However, because of poor resolution, peaks are frequently spread over a number of channels. Poor resolution can be the result of a number of factors, including noise produced by pulse-processing electronics and variation in a detector""s parameters. If the resolution of a detector is poor, it may be impossible to identify discrete peaks characteristic of a particular radioactive material because the peaks will simply blend together and be indistinguishable, thus making it difficult or impossible to accurately identify a corresponding radioisotope.
The resolution of a detector is often measured employing a full-width-at-half-maximum (FWHM) terminology. FWHM may be expressed as a ratio of the width of the peak at half of the peak""s maximum value over the peak""s maximum value. This ratio is frequently given as a percentage, and small values correspond to narrow peaks and good energy resolution.
Conventionally, thallium (Nal(Tl)) scintillators and high-purity germanium detectors have been used for in-field radioisotope analysis. Scintillators, however, suffer from poor resolution, resulting in a low-confidence level in data produce thereby. The resolution of a scintillator is typically about only 7% FWHM at 662 KeV.
While high-purity germanium detectors provide excellent resolution, they suffer from a number of serious disadvantages. These detectors require in-field calibration to ensure the accuracy of the readings taken. To calibrate these devices, of course, requires transportation of a radioactive material from which to gauge the detector. This is extremely cumbersome, dangerous, and frequently requires governmental licensing. Furthermore, high-purity germanium detectors must be cooled to liquid-nitrogen temperatures and are extremely fragile. Extensive training is required to correctly operate this type of detector.
Another concern with conventional portable radiation detectors deals with automated analysis to determine the composition of a corresponding radioactive material. Conventional techniques, such as the Chi-square analysis, are inflexible and do not consider the variables present with in field analysis. In-field analysis often involves unknown distances between the detector and radioactive material, an unknown form of the radioactive material (gas, solid, or liquid), and unknown barriers between the radioactive material and detector. Conventional techniques are rigid and, as such, may provide unsatisfactory results given these variables. Moreover, the conventional techniques are computationally intensive, particularly for the limited resources of portable devices. Thus, employing such techniques may draw substantial resources (e.g., battery power) from the portable detector and may not produce timely or accurate results.
Consequently, it would be an advancement in the art to provide a portable radiation detector having automated radioisotope identification capabilities sufficiently flexible to adapt to the variables of in-field analysis. It would be a further advancement in the art to provide a portable radiation detector which operates at room temperatures, does not require in-field calibration, is not fragile, does not require training to use, and yet provides higher resolution.
A handheld cadmium zinc telluride (CZT) radiation detector provides a portable radiation detector implementing a fuzzy-logic radioisotope identification procedure adapted to in-field analysis. This fuzzy-logic procedure is computationally less intensive and more flexible than conventional identification algorithms used in portable radiation detectors, thus providing timely and more accurate results to an end-user and extending the detector""s battery life. Furthermore, in one embodiment, the handheld CZT radiation detector implements a coplanar grid CZT gamma ray sensor which provides higher resolution than conventional scintillators, but does not require in-field calibration or cooling to liquid-nitrogen temperatures like high-purity germanium detectors. To be more specific, the handheld CZT radiation detector provides about twice the resolution of a conventional scintillator and may be operated by untrained individuals, unlike a high-purity germanium detector. In addition, as the name implies, the handheld CZT radiation detector is sized to be held in a person""s hand, just as a personal data assistant (e.g., a PalmPilot(copyright) by Palm, Inc.). Thus, the handheld CZT radiation detector is substantially smaller than conventional portable radiation detectors. In addition, the handheld CZT radiation detector is user-friendly because, in one embodiment, a user may interact with the detector through a personal data assistant.
The handheld CZT radiation detector may comprise a sensor for sensing gamma rays employing cadmium zinc telluride (CZT). The sensor may be coupled to a processor for processing commands stored in memory.
The sensor may be a CZT gamma-ray sensor. A CZT gamma-ray sensor may include any gamma-ray sensor using a CZT crystal. A CZT gamma-ray sensor may further comprise circuitry for converting charge produced by a gamma ray interacting with the CZT crystal into a shaped voltage pulse, which may be referred to as gamma-ray data. The amplitude of the shaped voltage pulse is proportional to the energy deposited on the CZT crystal by a gamma ray. CZT gamma-ray sensors are known, and those skilled in the art will understand that such sensors may be configured in numerous ways and still fall within the scope of this invention.
CZT gamma-ray sensors include, in one embodiment, a coplanar grid CZT gamma-ray sensor. The coplanar grid CZT gamma-ray sensor collects two separate anode signals from the CZT crystal. One signal indicates energy produced by charge motion in the CZT crystal and the other indicates the energy produced by both the charge motion and an interaction with a gamma ray. By subtracting the two signals using a differencing amplifier, the charge motion may be removed from the signal, producing excellent resolution. A shaping amplifier then modifies the pulse. Coplanar grid CZT gamma-ray sensors are known, and those skilled in the art are familiar with the numerous ways in which such sensors may be configured and still fall within the scope of this invention.
Use of a coplanar grid CZT gamma-ray sensor in a portable device results in a number of advantages over conventional portable radiation detectors. Coplanar grid CZT gamma-ray sensors operate correctly in a wide range of temperatures (e.g., about xe2x88x9210xc2x0 to about +50xc2x0 Celsius), unlike conventional high-purity germanium detectors, which operate correctly only at liquid-nitrogen temperatures. In addition, a coplanar grid CZT gamma-ray sensor provides far superior resolution to conventional scintillators.
The handheld CZT radiation detector may further comprise a multichannel analyzer (MCA) for performing multi-channel analysis. In one embodiment, the MCA is coupled to the sensor and processor. An MCA is configured to produce pulse height data corresponding to the gamma-ray data. Stated more precisely, the MCA receives a shaped voltage pulse (gamma-ray data) and converts the voltage pulse into a digital number indicating the height of the pulse. The MCA then categorizes the pulse into an energy level range (a channel). The MCA keeps track of the number of pulses (or counts) received in a given channel, producing count patterns, or pulse height data. Thus, the MCA is configured to produce pulse height data corresponding to the gamma-ray data.
The handheld CZT radiation detector may further comprise an analysis component for compiling a ranked listing of radioisotopes corresponding to the pulse height data. The radioisotopes are ranked by how closely they fit the pulse height data. The analyzer may be coupled to the MCA and processor.
In one embodiment, the analysis component may comprise a fuzzy-logic component. Counting statistics and measurement geometries can mask or hide usually prominent features in pulse height data. Crisp logic methods define criteria that categorize an event as either belonging to a set or not. Fuzzy logic does not involve such discrete categorizations, but involves an assigned weighting value, or possibility value, within a range of values. By way of example, if a peak within the pulse height data exhibits strong characteristics (i.e., the peak is tall relative to surrounding values and symmetrical) in may be assigned a high value, i.e., a 0.9 in a range of 0 to 1. A peak that does not exhibit such strong characteristics may be assigned a low value, such as 0.1.
Measurements taken by portable radiation detectors are subject to numerous variables, e.g., the distance between the radioactive material and the detector is unknown, the radioactive material is of an unknown form (gas, liquid, or solid) and an unknown geometry, and intermediary barriers may impede or vary the transmission of the gamma rays. The fuzzy logic component is better suited to adapt to the variables of in-field analysis than conventional in-field identification methods.
In one embodiment, the fuzzy logic component may comprise a peak search component, peak analysis component, energy-level component, matching component, library, and ranking component, or a subcombination thereof. The peak search component produces peak search data by analyzing pulse height data produced by the MCA. The peak search data contains information about possible peaks within the pulse height data.
The peak analysis component may analyze the peak search data and produce a weighting value indicating the significance and symmetry of a particular peak, resulting in a peak analysis weighting value. In one embodiment, the peak analysis component may comprise a peak significance component, a peak symmetry component, and a peak parity component. These three components may produce weighting values that may contribute to the peak analysis weighting value.
In one implementation, the library contains a listing of radioisotopes and characteristics of pulse height data corresponding to those radioisotopes. The energy-level component may use fuzzy logic to compare energy levels of peaks from the pulse height data produced by the handheld CZT radiation detector to peaks from radioisotopes in the library. An energy-level weighting value indicates how closely the energy levels of two such peaks match.
The matching component may use peak analysis weighting values and energy-level weighting values to produce a listing of radioisotopes, that most closely match the pulse height data produced by the handheld CZT radiation detector. In one embodiment, a ranking component will rank each radioisotope according to how closely it matches the pulse height data.
In one embodiment, the handheld CZT radiation detector may further comprise a neutron sensor to produce an indicator when neutrons are detected. The neutron sensor may comprise a helium-3 proportional counter. Helium-3 proportional counters are well-known and understood by those skilled in the art. The neutron detector provides an additional method of detecting the presence of radioisotopes when gamma rays may be shielded by a barrier.
The handheld CZT radiation detector may further comprise a display component coupled to the analysis component (e.g., a fuzzy-logic component) and the neutron sensor. In one embodiment, the display component comprises a personal data assistant for receiving the ranked listing of radioisotopes and the indicator and displaying a visual indication thereof. Incorporating a personal data assistant into this device makes the handheld CZT radiation detector more user-friendly, as individuals are generally familiar with personal data assistants.
An interface places the analysis component (e.g., a fuzzy-logic component) and neutron sensor in electrical communication with the display component. In one embodiment, the interface may comprise, for example, a serial port or an infrared port. The interface may convey a signal containing either the ranked listing of radioisotopes, the indicator of the presence of neutrons, or both to a display component. In one embodiment, the interface is configured for two-way communication, both to send and receive data.
In one implementation, the CZT, gammy-ray sensor, processor, memory, MCA, analysis component, neutron sensor, and interface, or a subcombination thereof may be received by the first housing. In such a configuration, the handheld CZT radiation detector is resistant to damage and easily transportable. The first housing may be sized to be held in a person""s hand. Sized to be held in a person""s hand means that the handheld CZT radiation detector can comfortably be held in one hand and operated with the other hand.
Also, the first housing may have a recess for receiving a display component, such as a personal data assistant, which may be situated within a second housing. In one embodiment, the personal data assistant may communicate with the sensor and neutron detector via the interface when the personal data assistant is placed in the recess.
These features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.